Based on Fabry-Perot (F-P) cavity interference, the fiber methane gas sensor system is studied. The micro-electromechanical systems (MEMS) technology is used to cladded cryptophane A polymer onto silicon-piece. The end faces of fiber constitute a F-P cavity. Based on the absorption of methane and gas-sensitive properties of cryptophane A polymer, by analyzing the interference peak wavelength changes, the relationship between concentration and wavelength is obtained. Sensing mechanism is analyzed theoretically and by sensing performance is evaluated by methane gas sensing experiment the validity of the methane gas is verified by methane gas samples. By measuring and analyzing data the information included in the interference signal is demodulated. The results show that, the sensor has high sensitivity and reliability, and the response time is faster.